This patent relates to a magnetoelectric device, particularly a memory and/or a logic element, and to a method for writing non-volatile information into said magnetoelectric device.
Developments in magnetoelectronics, also known as spintronics (also known as spin-based electronics) have being important in the recent years. Anisotropic magnetoresistance sensors (AMR) are being widely used to determine magnetic field strengths and/or its direction. Two other kinds of spintronics devices, memories and sensors, are under development.
An anisotropic magnetoresistance (AMR) sensor is a magnetic sensor based on the dependence of the electric resistance of a soft ferromagnetic layer on the angle formed by its magnetization and the direction of the measuring electric current.
For example, GMR (Giant Magnetoresistance) sensors are being used extensively in the read heads in modern hard drives and Magnetic Tunnel Junctions (MTJ) in non-volatile, magnetic random access memory (MRAM).
A GMR is made with at least two layers of ferromagnetic material separated by a non magnetic metal. FIG. 1A and FIG. 1B are schematic view of a basic GMR comprising two magnetic layers 100 and 102 separated by a metallic layer 104 in two different states.
In FIG. 1A, the magnetization 106 of the magnetic layer 100 and the magnetization 108 of the magnetic layer 102 are antiparallel. Then, the electric resistance of this component is high (High-Resistance state).
In FIG. 1B, after applying a magnetic field 110, the magnetization 108 of the magnetic layer 102 is aligned with the magnetization 106 of the magnetic layer 100. Then, the resistance of this GMR goes down (Low-Resistance state).
Similarly, a magnetic tunnel junction (MTJ) is formed, at least, by two layers of ferromagnetic material separated by a thin insulating layer. The electrical flow across the MTJ depends on the relative orientation of the magnetization of the ferromagnetic layers. Typically, when these magnetizations are parallel the resistance of the MTJ is in the Low-Resistance state whereas when the magnetizations are antiparallel, the resistance of the MTJ is in the High-Resistance state.
The distinct values of the Low and High-resistance states of the MTJ can be used to determine the magnetic state of the ferromagnetic electrodes. Also, changing the relative orientation of the ferromagnetic electrodes can modify the resistance state of the MTJ.
These addressable and readable two resistance states of a MTJ are therefore usable to store information (High resistance, Low-resistance; “1” or “0”). An electric current is needed to determine the High or Low resistance states of the MTJ. Electric power is not required to keep the state of the MTJ as it is only determined by the relative orientation of the magnetization of the electrodes. Information is thus remnant.
Information in MTJ is written by changing the orientation of the magnetization of one of the ferromagnetic layers (free layer) with respect to the other one, which is kept fixed (pinned). MRAMs comprise arrays of MTJ. The orientation of magnetization of the free magnetic layer is commonly controlled by an external magnetic field created by a pair of current-carrying wires (bit and word lines).
FIG. 1C shows schematically a known MRAM where information can be memorized producing a magnetic field in different points of a matrix using intersections of electrical conductors. For example, to write a ‘1’ in the place 160, a current 153 is created in the conductor 152 and a current 155 is created in the conductor 154, creating a maximum field in the place 160.
These MRAMs need power to be written because they need the current to create the magnetic field (so-called Oersted field) that will be used to change the field on a place of the device. Joule heating takes place because of the use of electric current to address the magnetic state of a MTJ. Energy dissipation is a drawback in terms of energy consumption from power supply and limits higher integration of MTJ in MRAM's.
And as it can be shown in FIG. 1C, when introducing currents 153 and 155 in order to create the magnetic field in the place 160 of a MRAM, it can also affect neighboring places 161, 162, 163 and 164 on the MRAM, changing their magnetizations too (problem known in the art as crosstalk).
Document WO2006/103065 discloses a magnetoresistive element and a method for writing information. The method is based on reported dependence of the exchange bias between adjacent ferromagnetic and magnetoelectric layers. Cooling the magnetoelectric across its antiferromagnetic order temperature (TN) under simultaneous application of suitable Electric and Magnetic fields (E and H) allows modifying the antiferromagnetic boundary condition and subsequently the exchange bias. The product of the field strengths of the two fields during freezing determines the ability to change the antiferromagnetic boundary condition. A characteristic of this WO2006/103065 known magnetoresistive element is that the need of maintaining the Electric field during the cooling will eventually lead to energy consumption